Preheat your oven to 350°F (175°C) and prepare two 9-inch round cake pans by greasing and dusting them with flour.

In a large mixing bowl, whisk together the dry ingredients: flour, sugar, cocoa powder, baking soda, baking powder, and salt.

In another bowl, mix the wet ingredients: eggs, milk, oil, and vanilla extract. Whisk until smooth.

Combine wet and dry mixtures, then gradually stir in boiling water until batter is smooth.

Divide the batter between the prepared pans and bake for 30-35 minutes, checking for doneness with a toothpick.

Cool the cakes in the pans for 10 minutes before transferring to wire racks to cool completely.

Beat together softened butter and peanut butter in a medium bowl until fluffy. Gradually add powdered sugar and mix until combined.

Heat heavy cream in a saucepan until steaming, then stir in chopped semisweet chocolate and whisk until smooth.

Place one cooled cake layer on a serving plate and spread half of the peanut butter frosting on top.

Add the second layer and frost the top and sides of the cake with remaining frosting.

Pour ganache over the frosting, allowing it to drizzle down the sides. Refrigerate for at least 1 hour before serving.